1. Technical Field
The present invention relates to a liquid crystal display, in which an afterimage continued to be displayed for a certain time after the turning-off of the power is removed by the load capacitance possessed by liquid crystal display elements in a liquid crystal display panel.
2. Prior Art
A TFT type LCD having active switching elements has a structure in which many active switching elements are arranged in a matrix. FIG. 1 shows an equivalent circuit of such TFT. One end of the TFT is connected to a data line, and the other end is connected to a pixel voltage terminal Vp. And, its gate is connected to a gate line N, and the TFT is selectively turned on/off in response to the signal provided to the gate line N. Further, the pixel voltage terminal Vp is connected to a voltage terminal VCOM through a liquid crystal capacitance CLC and connected to a gate line N-1 through a secondary capacitance CS.
Generally, the output from a driver circuit for driving such panel is interrupted simultaneously with the turning-off of the power. Thus, even if the power is turned off to eliminate the display on a screen, sometimes an afterimage like a black spot is seen on the screen for a certain time. The occurrence of such afterimage is caused by the charge held on a load capacitance comprised of the liquid crystal capacitance CLC and the secondary capacitance CS. That is, while the charge is naturally discharged after the power is turned off, it is made visible on the screen by a reflected light as if it is written. Since the degree of the afterimage made visible by the reflected light increases as Aperture ratio of the TFT type LCD increases, it is a problem from the viewpoint of the display screen quality.
To solve the problem of the afterimage display, it is only required that, when the power is turned off, all the gate lines (all the unselected gate lines) supplied with a gate low voltage (for instance, -10 V,) by a gate line driver circuit be supplied with a voltage of the order of +2 V for a certain time period ever after the turning-off of the power. Since the TFTs in the corresponding active switching elements are caused to become a low-resistance state (on-state) by providing a voltage to the unselected gate lines to such extent, the charge in the pixel capacitance can forcibly be discharged to the gate lines on the instant. Accordingly, it is possible to instantaneously clear the afterimage.
As a background art related to this problem, there is Published Unexamined Japanese Patent Application No. 1-170986. This is to automatically detect the power-off of a liquid crystal display, and based on that, the TFT of the liquid crystal display element is held to be on for a predetermined time so that the stored charge of the pixel capacity can be discharged in a short time. As shown in the block diagram of FIG. 2, by a source voltage Vl supplied to a terminal 1 from the main body of a liquid crystal display, a capacitor 2b of a large capacitance is charged through a diode 2a. The source voltage Vl is also supplied to a gate bus driver circuit 3. This source voltage holding circuit 2 is to hold the power of the operation power supply provided to a liquid crystal display panel for a predetermined time even after the power to the liquid crystal display is turned off. A voltage drop detection circuit 4 is to detect the voltage drop of the source voltage Vl, and simultaneously holds the outputs of the gate bus driver circuit 3 at an active level for a predetermined time in response to the detected signal.
That is, in this method, when the system power Vl is turned off, the power within the LCD panel is not immediately turned off, but, first the voltage drop from the system is detected and a voltage supply circuit for rising the gate voltage is operated. Thereafter, after the afterimage on the screen is removed, to turn off the main power to the LCD panel, instructions are sent to the power circuit to finally turn off the power.
However, the background art has the following problems. First, since the source voltage Vl is supplied to the gate bus driver circuit 3 through the diode 2a, a voltage drop occurs in the diode 2a. To solve the disadvantage due to the voltage drop, the above publication includes a description that the voltage Vl itself may be made larger or a DC-DC converter may be provided on the input side of the power holding circuit 2 to increase the voltage. However, it cannot be denied that the circuit design is made complicated by the provision of such circuit.
Further, as a larger essential problem, it is required to reserve a power supply for operating circuits such as a circuit for detecting the turn-off of the system power, or an afterimage removal power hold/supply circuit and a power turn-off request/enable circuit after the afterimage removal, which are needed in the above art, after the system power Vl is turned off. However, circuit construction therefor is complex and expensive.